What is your antivirus?Can you add the Steam folder to exceptions in the antivirus?You will have to go through your antivirus to get to Steam, if it is indeed stopping you from launching.If you can't do anything, I suggest changing antivirus.

I think these files are necessary for the steamui dll to function so that is why it won't run.

Repost if uninstalling and reinstalling works... Steam on my PC was working just fine as well when I left the house this afternoon. If it doesn't fix it then I think the folks at Valve might have to work it out. Obviously an automatic update to Steam wouldn't help because it isn't launching for us. :/

Side note: I attempted to repair Steam via the SteamInstall.msi off the website but apparently the Support.url file the Cabs.w1.cab file couldn't be located or was corrupted so the installation process ended prematurely due to that error.

My AV is Kaspersky 2013. Steam gave me a minor update today sometime and discovered the trojan.win32.Agent.unnn. According to thier site they just dicovered this malware just after midnight on nov 3 2012. That's just a few hours ago. I had to make sure my AV database was up to date, run a scan and was still finding a threat, so I uninstalled steam and rescanned and that seemed to do the trick.

Showed up on Kaspersky for me too. I've submitted two false positive reports. I've added Steam to the exclusion folder until Kaspersky can get it sorted out :/ Steam/L4D2 works fine after adding to the exclusion list.

I just got this happen to me about 30 minutes ago. ZoneAlarm extreme. Trojan.Win32.Agent.unnnTrojan.Win32.Agent.unncIt was unable to delete them and quarantined them. Steam updates and the files are found again with virus.

Either its a false flag or steam is updating everyone with a trojan =)

I just got this happen to me about 30 minutes ago. ZoneAlarm extreme. Trojan.Win32.Agent.unnnTrojan.Win32.Agent.unncIt was unable to delete them and quarantined them. Steam updates and the files are found again with virus.

Either its a false flag or steam is updating everyone with a trojan =)